The present invention relates to a mechanical compensating device for a waveguide (1). More precisely, the present invention provides a technology for ensuring phase stability in a waveguide (1) subject to expansions and contractions owing to temperature changes. To do this, actuators, which may consist of pairs of prongs (8-9, 10-11), connected to longitudinal ribs (2, 3) cut in the body of the waveguide (1) and integral therewith, cause, because of a large difference between the respective coefficients of thermal expansion of the waveguide (1) and of the actuators, a rotation of the longitudinal ribs (2, 3) about themselves, deforming the short sides (4, 5) of the waveguide (1) when said waveguide (1) expands or contracts according to the changes in temperature.